1. Field of the Invention
This invention relates to a method and an apparatus for recovering waste heat from electric furnaces whose exhaust gases have a high dust content or are strongly corrosive at relatively low temperatures. The recovery of waste heat from such furnaces has been very difficult with the conventional techniques.
2. Description of the Prior Art
Various devices have been proposed heretofore for recovering waste heat from varous kinds of electric furnaces such as carbide furnaces, ferroalloy furnaces and electric smelting furnaces. In this context, the recovery of waste heat does not mean simply cooling the furnace to prevent overheating of structural parts or for other operational purposes, but rather means recovering useful heat that would otherwise be lost.
Among the attempts for heat recovery employed in the past there are, for example, a method in which the hightemperature atmosphere zone (which may reach a temperature of several hundred degrees centigrade) in the upper portion of the electric furnace is invested by a water-cooled double wall structure and the used cooling water is heated to a temperature of over 100.degree. C whereby the heat is recovered in the form of hot water or steam, and a method in which dust-laden waste gas having a temperature of several hundred degrees centigrade is introduced directly into a hot water generator or steam generator.
According to these methods, however, the temperature of the electric furnace discharge gas fluctuates widely when the internal pressure of the waste heat recovering device is kept lower than atmospheric pressure, and particularly in the case of a blowing-in type of furnace, the exhaust gas temperature tends to rise sharply. For these reasons, the temperature of the recovered cooling water is usually lowered to on the order of 40.degree. to 50.degree. C, resulting in the recovery of an extremely low quantity of useful heat. In order to increase the quantity of useful heat recovered, the internal pressure of the recovery device must be kept higher than atmospheric pressure, but in such a case, proper maintenance and management for safe operaton of the device become very troublesome and also such apparatuses are subject to various legal regulations and controls relating to "boilers" or "pressure vessels." Therefore, periodic shutdown of the furnace is required for maintenance and inspection of the waste heat recovering apparatus, resulting in substantially reduced production of the electric furnace itself.
Another difficulty is that the dust contained in exhaust gas is generally very liable to adhere to the structural parts and because this dust has poor heat conductivity, it is required that the heat transfer area of the waste heat recovery apparatus be increased, thereby resulting in an enlarged size of the apparatus as a whole. If a multitubular heat exchanger or the like is employed for reducing the general size of the apparatus, largescale and expensive equipment and troublesome operations are required for removing the dust which builds up heavily in such heat exchanger or the like.
The present invention provides a waste heat recovering apparatus which is free of the aforementioned disadvantages of the prior art and is capable of maximizing the recovery of useful heat without reducing the production capacity of the electric furnace itself.
More specifically, the waste heat recovering apparatus according to the present invenion, which is designed for use in conjunction with electric furnaces of the type that discharge high-temperature gas laden with a large amount of dust, employs primary heat recovery using water or other cooling liquid at a temperature below the boiling point thereof and under atmospheric pressure, and secondary heat recovery practiced by passing the dust-laden discharge gas, after primary heat recovery, through a hollow structure whose internal surfaces are of such nature as to repel adhesion of dust thereto and/or to allow easy removal of any dust that may be deposited thereon, while simultaneously passing air or other gas as a heat exchange medium along the opposite side wall at a temperature higher than the boiling point of water. The invention is further characterized by the fact that the entirety or part of the water or other liquid which has undergone said primary heat recovery is heated with air or other gaseous medium which has undergone the secondary heat recovery by using a heat exchanger or boiler, whereby to obtain a high-temperature liquid or steam.